CN112442629B - Medium-carbon steel for mechanical structure and manufacturing method thereof - Google Patents

Medium-carbon steel for mechanical structure and manufacturing method thereof Download PDF

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CN112442629B
CN112442629B CN201910801797.8A CN201910801797A CN112442629B CN 112442629 B CN112442629 B CN 112442629B CN 201910801797 A CN201910801797 A CN 201910801797A CN 112442629 B CN112442629 B CN 112442629B
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steel
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CN112442629A (en
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郑宏光
柳向椿
刘俊江
刘耀宗
孟庆玉
马强
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Baoshan Iron and Steel Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/10—Supplying or treating molten metal
    • B22D11/11—Treating the molten metal
    • B22D11/114—Treating the molten metal by using agitating or vibrating means
    • B22D11/115—Treating the molten metal by using agitating or vibrating means by using magnetic fields
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/072—Treatment with gases
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

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  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

The medium carbon steel for mechanical structure and the manufacturing method thereof comprise the following components by mass percent: 0.40-0.54% of C, 0.27-0.35% of Si, 0.6-0.8% of Mn, less than or equal to 0.025% of P, less than or equal to 0.015% of S, less than or equal to 0.005% of N, 0.015-0.05% of Al, less than or equal to 0.001% of O, 0.06-0.2% of V, 0.2-1.0% of Hf, 0.001-0.005% of Ca, and the balance of Fe and inevitable impurities, and also contains up to 1% of optionally added rare earth and reactive elements such as Ce, La, Re, Sc and/or Y. The invention adopts the microalloy addition design, and under the condition of adding V, the lower content of total oxygen is controlled by adding a proper amount of Hf (hafnium), Ca (calcium) and the like, so that the material has higher strength, and the trace elements of hafnium and calcium are added to further refine the crystal grains of the steel and form harder particles in the steel, thereby not only improving the strength, but also improving the plasticity.

Description

Medium-carbon steel for mechanical structure and manufacturing method thereof
Technical Field
The invention belongs to the field of steel for a mechanical structure, and mainly relates to medium carbon steel for the mechanical structure and a manufacturing method thereof.
Background
The 50CrV steel grade is used as a high-load important spring with a larger cross section, and a valve spring, a piston spring and a safety valve spring with the working temperature of less than 300 ℃. The general use specification is between 1.5 and 10.0mm, and the annealing state is more. The steel coil is in a hot rolling state, and spheroidizing annealing is adopted to reduce the hardness so as to create conditions for further processing. The steel is generally an annealed plate in a delivery state, and the treatment of re-annealing is omitted.
TABLE 1 chemical composition in wt% of the relevant typical steel grades
Figure GDA0003362374360000011
TABLE 2 mechanical Properties of typical steels of interest
Figure GDA0003362374360000012
As can be seen from tables 1 and 2, with the advancement of the technology, the practical application puts higher demands on the steel grade. The steel for mechanical structure in the prior art can not completely meet the current use and manufacture requirements, and needs to be developed with higher (tensile strength and yield strength), better plasticity (elongation percentage and reduction of area) and more reasonable cost.
In the prior art, the strength of steel grades is designed mainly by adding elements such as Si, Mn, Cr (Ni) and the like for solid solution strengthening, and VN (vanadium nitride) is precipitated at grain boundaries by adding elements such as V, N and the like for achieving the effect of strengthening the grain boundaries. With the technological progress, higher demands are made on the strength of this steel grade. In order to further improve the strength, a feasible and new strength design idea needs to be provided to meet the requirement of technical progress.
Disclosure of Invention
The invention aims to provide a medium carbon steel for mechanical structure and a manufacturing method thereof, the alloy has better mechanical property and process property, and the yield strength is more than or equal to 1150 MPa; the tensile strength is more than or equal to 1300MPa, the hardness performance is good (HB is more than or equal to 265), the reduction of area is more than or equal to 40 percent, the material cost is relatively low, and the material is mainly used for manufacturing high-load important springs with larger cross sections, and valve springs, piston springs and safety valve springs with the working temperature of less than 300 ℃.
In order to achieve the above objects, the technical solution of the present invention is,
the invention adopts the microalloy addition design, and controls the lower content of total oxygen by adding a proper amount of Hf (hafnium), Ca (calcium) and the like under the original condition of adding V, so that the material has higher strength, and the trace elements of hafnium and calcium are added to further refine the crystal grains of the steel and form harder particles in the steel, thereby not only improving the strength, but also improving the plasticity.
Specifically, the medium carbon steel for mechanical structures comprises the following components in percentage by mass: 0.40 to 0.54 percent of C, 0.27 to 0.35 percent of Si, 0.6 to 0.8 percent of Mn0.6, less than or equal to 0.025 percent of P, less than or equal to 0.015 percent of S, less than or equal to 0.005 percent of N, 0.015 to 0.05 percent of Al, less than or equal to 0.001 percent of O, 0.06 to 0.2 percent of V, 0.2 to 1.0 percent of Hf, 0.001 to 0.005 percent of Ca0.001 to 0.005 percent of Al, and the balance of Fe and other inevitable impurities, and simultaneously satisfy the following conditions:
also contains one or more of Ce, La, Re, Sc and Y, the content of which is not more than 1%;
Hf/C＝0.5-1.5、Hf/N＝60-140；
Ca/O＝1-5、Ca/S＝0.06-0.6。
preferably, the content of V is 0.08-0.1% by mass.
Preferably, the content of Hf is 0.3-0.7% by mass percentage.
Preferably, the content of Ca is 0.001 to 0.003% by mass.
Preferably, the steel for machine structural use has a microstructure of ferrite + pearlite and a grain size of 12 to 22 μm.
In the composition design of the steel of the invention:
c: generally, the C content mainly affects the precipitation amount of carbides and the precipitation temperature range. The control of the lower C content is beneficial to improving the mechanical property of the steel. Carbon has a certain strengthening effect, but too high a carbon content may reduce the corrosion resistance of the material. The production capacity of the current smelting equipment can meet the requirement of controlling carbon within a required range. Thus, the material mechanical property and the impact toughness are favorably considered.
N: is a stable austenite element. Controlling the N content lower is beneficial to improving the impact toughness of the steel. High nitrogen content results in reduced toughness and ductility and reduced hot workability. Therefore, the upper limit is 0.05%. The nitrogen content in the steel can be controlled by adopting the high molten iron ratio electric furnace smelting process.
Si: the strength can be improved in steel, but is disadvantageous in formability and toughness of steel. The element often remains in the smelting process, so that it is important to properly select the content.
Mn: weaker austenite elements can inhibit the harmful effect of sulfur in steel for machine structural use, and improve the thermoplasticity. However, too high a content is disadvantageous for securing corrosion resistance. The element often remains in the smelting process, and the content of the element needs to be properly selected.
Al: the alloy is strengthened mainly by controlling the oxygen content in the steel to influence the dislocation behavior. Increasing the total amount of aluminum can significantly increase the solution temperature, mechanical properties, but the plasticity is compromised. The aluminum is beneficial to the extension deformation performance of the steel and improves the processing performance of the steel. Too high an aluminum content may reduce the impact toughness of the steel.
O: is an impurity element in steel and mainly exists as oxide inclusion, and the total oxygen content is high, which indicates that the inclusion is more. The total oxygen content is reduced, which is beneficial to improving the comprehensive performance of the material. The total oxygen is controlled to be less than or equal to 0.001 percent, and the good mechanical and corrosion resistance of the material can be ensured.
P and S: they can seriously affect the mechanical properties and the processability of the steel for low temperature use and must be strictly controlled. Usually, P is controlled to be less than or equal to 0.010 percent and S is controlled to be less than or equal to 0.005 percent.
Cr: chromium increases the hardenability of the steel and has a secondary hardening effect, which increases the hardness and wear resistance of the carbon steel without embrittling the steel. The chromium can improve the strength and hardness of the carbon steel in a rolling state and reduce the elongation and the reduction of area. The main function of chromium in the steel with a quenched and tempered structure is to improve hardenability, so that the steel has better comprehensive mechanical properties after quenching and tempering, and chromium-containing carbide can be formed in carburizing steel, thereby improving the wear resistance of the surface of the material. The spring steel containing chromium is not easily decarburized during heat treatment. The chromium can improve the wear resistance, hardness and red hardness of the tool steel and has good tempering stability.
V: has strong affinity with carbon, oxygen and oxygen, and can form corresponding stable compounds. Mainly in the form of carbides in the steel. The main function of the steel is to refine the structure and the crystal grains of the steel and reduce the strength and the toughness of the steel. When the solid solution is dissolved at high temperature, the hardenability is increased; conversely, if present in the carbide form, the hardenability is reduced. Vanadium increases the temper stability of the quenched steel and produces a secondary hardening effect. The vanadium content in the steel is generally not more than 0.5 percent except for high-speed tool steel. The vanadium can refine crystal grains in the common low-carbon alloy steel, improve the strength, yield ratio and low-temperature characteristics after normalizing and improve the welding performance of the steel. Vanadium is often used in combination with elements such as manganese, chromium, molybdenum, and tungsten in structural alloy steels because it reduces hardenability under ordinary heat treatment conditions. In the quenched and tempered steel, vanadium mainly improves the strength and yield ratio of the steel, refines grains and has overheating sensitivity. The crystal grains can be refined in the case of carburizing steel, so that the steel can be directly quenched after carburizing without secondary quenching. Vanadium can improve the strength and yield ratio, especially the proportion limit and the elastic limit, in spring steel and bearing steel, and reduce the decarburization sensitivity during heat treatment, thereby improving the surface quality.
Hf: hafnium is a strong carbide former, and Hf alloys can be used to make tool steels and electrical resistance materials. Hafnium is used as an additive element in the heat-resistant alloy, and is added to alloys of, for example, tungsten, molybdenum, and tantalum. HfC can be used as a hard alloy additive due to its high hardness and melting point. Hafnium can act as a getter for many gas-filled systems. Hafnium getters can remove unwanted gases such as oxygen, nitrogen, etc. present in the system.
Ca: is a very active metal element. Has strong affinity with oxygen, nitrogen and sulfur. Therefore, calcium is a good deoxidizing and desulfurizing agent in steel smelting, and calcium added into steel can refine grains, deoxidize and desulfurize, and change the components, the quantity and the form of non-metallic inclusions; the corrosion resistance, the wear resistance, the high temperature resistance and the low temperature resistance of the steel are improved; the plasticity, impact toughness, fatigue strength and welding performance of the steel are improved; and can also enhance the hot cracking resistance, the hydrogen induced cracking resistance and the lamellar resistance of the steel.
The steel for the mechanical structure designed by the invention has a ferrite plus pearlite structure at room temperature, and the crystal grain is finer than that of the existing steel grade. It has good mechanical property and processing property, etc.; the method is more suitable for manufacturing high-load important springs with larger cross sections, and valve springs, piston springs and safety valve springs with the working temperature of less than 300 ℃.
Compared with the related typical steel grades, the invention has the main characteristics that:
1. a small amount of Hf is added, part of Hf is dissolved in the steel grade in a solid manner, and a proper amount of HfC is formed in part, so that the strength of the steel is improved, the crystal grains of the steel are refined, and the stamping performance is improved;
2. the Ca element is added in a small amount, so that the harm of oxygen and sulfur in the steel to grain boundaries can be inhibited, thereby refining grains and improving the impact toughness of the steel.
The invention discloses a theoretical basis and a design idea for adjusting alloy components:
adding a small amount of Hf element into the steel grade of the invention, wherein part of the element is dissolved in the steel in a solid mode, and the other part of the element forms a certain amount of HfC particles, and the amount of Hf element added needs to satisfy the following quantitative relation: Hf/C is 0.5-1.5, Hf/N is 60-140, so that it is favorable for forming a certain quantity of HfC particles (can effectively raise strength of steel), and can inhibit N diffusion in the steel in a certain range so as to obtain the effects of further raising strength of steel, refining crystal grain and improving punching property.
Secondly, a small amount of Ca element is added into the steel grade of the invention, and the element can be combined with O, S element in the steel, and the following quantitative relation is required to be satisfied: Ca/O-1-5, Ca/S-0.06-0.6. Therefore, a certain amount of CaO and CaS can be formed in the process of molten steel solidification, on one hand, oxides and sulfides in steel can be reduced, and the weakening effect of oxygen and sulfur on grain boundaries is inhibited; on the other hand, the particles can also play the roles of grain refinement and austenite grain stabilization, and are beneficial to improving the impact toughness of the material.
The manufacturing method of the steel for the mechanical structure comprises electric furnace steel making, LF and VD/RH refining, square billet continuous casting, primary rolling and cogging, billet coping, secondary rolling and forming, quenching and tempering, or annealing and pickling; wherein the content of the first and second substances,
adding a silicon-calcium alloy at the last stage of VD/RH refining, blowing argon gas and stirring after the components are qualified, and controlling the flow of the argon gas to be 5-8 liters/minute; adding hafnium-iron (Hf) into a continuous casting tundish, and arranging an electromagnetic stirring device in the continuous casting tundish to uniformly stir molten steel in the tundish;
continuously casting a square billet with the thickness of 300 multiplied by 400-500 multiplied by 600 mm;
and the initial rolling and cogging are carried out at the heating temperature of 1150-1250 ℃ and the compression ratio (original section area/post-cogging section area) is controlled to be 2.5-5.0.
And in the secondary rolling, the billet is reheated to 1150-1250 ℃, and the compression ratio is controlled to be 1.0-4.0.
Preferably, in the square billet continuous casting procedure, the drawing speed is controlled to be 0.45-0.95 m/min; crystallizer covering slag is adopted.
Preferably, a crystallizer is adopted for electromagnetic stirring in the continuous casting square billet, the current is 200-500A, and the frequency is 2.5-5.5 Hz; the equiaxed crystal proportion of the cross section of the continuously cast square billet is more than or equal to 20 percent.
Preferably, in the quenching and tempering, the quenching heating temperature is 850-880 ℃, the cooling speed is controlled to be 50-100 ℃/S, the tempering heating temperature is 640-670 ℃, and the cooling speed is controlled to be 50-100 ℃/S.
In the method for manufacturing a steel for machine structural use according to the present invention:
adding silicon-calcium alloy (Ca) at the final stage of VD (RH) refining, and after the components are qualified, stirring by blowing argon, wherein the flow of the argon is controlled to be 5-8 liters/minute. A small amount of hafnium iron (Hf) is added into the continuous casting tundish to meet the component requirement. The continuous casting tundish is provided with an electromagnetic stirring device for uniformly stirring the molten steel in the tundish.
In the continuous casting procedure, the drawing speed is controlled to be 0.45-0.95 m/min; crystallizer covering slag is adopted.
Preferably, a crystallizer is adopted for electromagnetic stirring in the continuous casting square billet, the current is 200-500A, and the frequency is 2.5-5.5 Hz; the equiaxed crystal proportion of the cross section of the continuously cast square billet is more than or equal to 20 percent.
The continuous casting bloom is subjected to surface finishing and coping to remove visible surface defects and ensure good surface quality.
The size of a sample blank for testing the mechanical property of the steel is 25mm, and after the sample blank is hot rolled into the steel for the second time, the hot rolled steel obtains good mechanical property by controlling quenching and tempering. This is advantageous for improving the room temperature mechanical properties and impact toughness. Obtaining the hot rolled product with comprehensive performance meeting the requirement.
Hf and Ca in the steel are mainly realized by adding hafnium-iron alloy and pure calcium alloy.
Can form an average diameter of 0.3-2 μm and a number of 5-10 pieces/mm in the alloy during the cooling solidification process2The HfC mass point has a diameter of 0.3-8 μm and a number of 2-15 particles/mm2CaO/CaS particles.
The particles can refine and stabilize the grain size of the alloy in the continuous casting solidification process, the hot rolling process, the heat treatment process and the like, not only can improve the mechanical property of the alloy, but also can avoid the formation of metallurgical defects on the surfaces of bloom and hot rolled products.
Compared with the existing similar alloy structural steel, the invention has the following beneficial effects:
the invention has two kinds of microalloy addition in the component design: hf and Ca are added, a certain amount of HfC can be promoted to be formed in the alloy in the solidification process, mass points can play a strengthening role, and the diffusion of N in steel can be inhibited; ② certain amount of CaO and CaS particles are formed in the solidification process. Thus, a diameter of 0.3 to 8 μm and a number of 5 to 10 pieces/mm are formed2HfC and a diameter of 0.3-8 μm and a number of 2-15/mm2CaO/CaS particles. These particles can serve to refine and stabilize austenite grains.
By adopting the technology, the alloy can control the grain size of the final product of the invention to be 12-22 μm (8.0-9.5 grade), and has obvious improvement compared with the prior art, thereby being very beneficial to improving the mechanical property of the material; the prior art can only control the grain size to be 27-37 mu m (6.0-7.5 level).
In comparison, the existing similar steel grades do not have the design, and for some applications, the mechanical property and the impact toughness cannot meet the requirements.
Detailed Description
The present invention will be further described with reference to the following examples.
According to the chemical composition requirements of the steel grade, a steel billet is subjected to electric furnace smelting, LF furnace and VD (RH) refining, soft stirring is carried out for a period of time, Hf + Ca is added, a bloom continuous casting machine is adopted for casting, and finishing and grinding are carried out on a large square plate blank at room temperature; the hot rolling comprises primary rolling cogging and secondary rolling, the heating temperature is 1150-1250 ℃, a large square billet is continuously cast, and the billet is cogging into a billet with a middle section; then, reheating the steel billet to prepare round steel or square steel with the thickness of 25-250 mm; the quenching heating temperature of the hot rolled round steel or square steel is 850-880 ℃, the cooling speed is controlled at 50-100 ℃/S, the tempering heating temperature is 640-670 ℃, and the cooling speed is controlled at 50-100 ℃/S.
The comparative steel grade used conventional compositions. The product is used at room temperature to 100 deg.C for a long time, and the preparation process is carried out by heat treatment. The performance tests of the comparative example and the embodiment of the invention adopt steel with the diameter of 10 mm.
The specific compositions of the inventive example steels and the comparative example steels are shown in table 3, table 4 shows the manufacturing process parameters of the inventive example steels, and table 5 shows the mechanical properties of the inventive example steels and the comparative example steels.
Figure GDA0003362374360000081
Figure GDA0003362374360000091

Claims (13)

1. The medium carbon steel for the mechanical structure comprises the following components in percentage by mass: 0.40 to 0.54 percent of C, 0.27 to 0.35 percent of Si, 0.6 to 0.8 percent of Mn0.6, less than or equal to 0.025 percent of P, less than or equal to 0.015 percent of S, less than or equal to 0.005 percent of N, 0.015 to 0.05 percent of Al, less than or equal to 0.001 percent of O, 0.06 to 0.2 percent of V, 0.2 to 1.0 percent of Hf, 0.001 to 0.005 percent of Ca, and the balance of Fe and other inevitable impurities, and simultaneously satisfy the following conditions:
also contains one or more of Ce, La, Re, Sc and Y, the content of which is not more than 1%;
Hf/C＝0.5-1.5、Hf/N＝60-140；
Ca/O＝1-5、Ca/S＝0.06-0.6。
2. the steel for medium carbon mechanical structure according to claim 1, wherein the content of V is 0.08 to 0.1% by mass.
3. The steel for medium carbon mechanical structure according to claim 1, wherein the content of Hf is 0.3 to 0.7% by mass.
4. The steel for medium carbon mechanical structure according to claim 1, wherein the content of Ca is 0.001 to 0.003% by mass.
5. The steel for medium carbon mechanical structure according to claim 1 or 2 or 3 or 4, wherein the microstructure of the steel for mechanical structure is ferrite + pearlite and the grain size is 12 to 22 μm.
6. The steel for medium carbon mechanical structure according to claim 1 or 2 or 3 or 4, characterized in that the steel for mechanical structure has a yield strength of 1150MPa or more, a tensile strength of 1300MPa or more, a hardness property HB of 265 or more and a reduction of area of 40 or more.
7. The steel for medium carbon mechanical structure according to claim 5, wherein the steel for mechanical structure has a yield strength of 1150MPa or more, a tensile strength of 1300MPa or more, a hardness property HB of 265 or more and a reduction of area of 40 or more.
8. The method for producing a steel for medium carbon mechanical structure as set forth in any one of claims 1 to 4, comprising electric steelmaking, LF and VD/RH refining, billet continuous casting, blooming, billet coping, secondary rolling into a material, quenching + tempering or annealing + pickling; wherein the content of the first and second substances,
adding a silicon-calcium alloy at the last stage of VD/RH refining, blowing argon gas and stirring after the components are qualified, and controlling the flow of the argon gas to be 5-8 liters/minute; adding hafnium-iron (Hf) into a continuous casting tundish, and arranging an electromagnetic stirring device in the continuous casting tundish to uniformly stir molten steel in the tundish;
continuously casting a square billet into a large square billet with the diameter of 300 multiplied by 400-500 multiplied by 600 mm;
the initial rolling and cogging are carried out, the heating temperature is 1150-1250 ℃, and the compression ratio is controlled to be 2.5-5.0;
and (3) performing secondary rolling to obtain a material, namely reheating the billet to 1150-1250 ℃, and rolling the billet into round steel or square steel with the thickness of 25-250 mm.
9. The method for producing a steel for medium carbon mechanical structure according to claim 8, wherein in the billet continuous casting step, the casting speed is controlled to 0.45 to 0.95 m/min; crystallizer covering slag is adopted.
10. The method for manufacturing steel for medium carbon mechanical structure as set forth in claim 8, wherein the continuous casting billet is electromagnetically stirred by a crystallizer, the current is 200-500A, and the frequency is 2.5-5.5 Hz; the equiaxed crystal proportion of the cross section of the continuously cast square billet is more than or equal to 20 percent.
11. The method for manufacturing steel for medium carbon mechanical structure as set forth in claim 8, wherein in the quenching and tempering, the quenching heating temperature is 850-.
12. The method for producing a steel for medium carbon machine structural use according to claim 8, wherein the steel for machine structural use has a microstructure of ferrite + pearlite and a grain size of 12 to 22 μm.
13. The method for producing a steel for medium carbon mechanical structure according to claim 8 or 12, wherein the steel for mechanical structure has a yield strength of not less than 1150MPa, a tensile strength of not less than 1300MPa, a hardness property HB of not less than 265 and a reduction of area of not less than 40%.
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